Mercruiser 5.7L backfire
 

All,

I need some help. I have a 1998 214 Jubilee with a 5.7L Mercruiser with Thunderboat 5 ignition. The boat starts and idles fine. When I first start the motor and let it warm up to running temp. I can open up the boat and run without any problems for about 10-15 minutes. After that the motor will backfire through the carb. It runs fine at low rpm but once I get over 2000 closer to 2500 rpm the boat will backfire. I have brought the boat into my local mercruiser dealer and we have not been able to overcome the problem, even with tech support from Mercruiser. Here is a list of things that have been looked at and tried.

New fuel filter, sea foam in gas, drained gas and replaced with new fuel, fuel pump, knock sensor, knock module, ignition module, new carb. Essentially all of the electronics have been replaced by the recommendation by Mercruiser. Inspected the distributer cap for cracks, used salt water to see if there is any arcing in the plug wires with no problems. I am sure there is more but I cannot remember everthing they have tried. The dealer has more or less given up on the project and have given me a large bill without fixing the problem.

If anybody has any suggestions that would be greatful.

Thanks in advanced.

David

Re: Mercruiser 5.7L backfire
 

Replace the coil. An aging coil can cause these problems. It will lead to an insufficient spark at mid-high rpm. It may also run a little rough the first time you take it out as the plugs will likely still be somewhat fouled but it should clear itself. Alternatively, clean (or replace) the plugs while you are replacing the coil .

Re: Mercruiser 5.7L backfire
 

Mike,

Thanks for the input. Is the coil very hard to replace? Or should this be done by the dealer?

Re: Mercruiser 5.7L backfire
 

Should be relatively easy. Look for where the center wire from the top of the distributor cap goes. At the other end of that will be your coil.

Jim

Re: Mercruiser 5.7L backfire
 

+1 on what Jim said. :wink_thumbup:

Re: Mercruiser 5.7L backfire
 

Thanks Jim & Mike. I will have to get one ordered. Do you know of a good website to order mercruiser parts from?

Re: Mercruiser 5.7L backfire
 

iboats.com works for me but there are many others including your local mercruiser dealer.

Re: Mercruiser 5.7L backfire
 

had a 95 did the same thing with 2 cracks in the head intake seat

Re: Mercruiser 5.7L backfire
 

Does anybody have any more thoughts on the problem? We tried a new coil, temp sensor, and temp sender unit this week and still have the same problems.

They are thinking it is internal now, possibly a sticking valve.

Re: Mercruiser 5.7L backfire
 

have you done a compresion ck hot and cold. Look at the plugs and make sure one of them is not sutted up. It may have exhaust backpresure in one of the cyl. Should show on plug if not it may be the cracked cyl head as I stated before. The cam lobes appear to be ok? I am a 30 year engine builder for stock susp drag and heads up drag. If I can help you can call me . There is nothing that I have not seen or done to an engine it is easier to talk you through it than to type.

Wendell 256 245 9214